Hydrochloric acid-catalyzed coproduction of furfural and 5-(chloromethyl)furfural assisted by a phase transfer catalyst

Article abstract of DOI:10.1016/j.carres.2020.108105

Furfural has been produced in 53percent isolated yield from D-xylose within an aqueous HCl-1,2-dichloroethane biphasic reaction mixture using benzyltributylammonium chloride (BTBAC) as a phase transfer catalyst. The use of BTBAC noticeably improved the yield of furfural compared to that in the control reaction. The reaction was optimized on the reaction temperature, duration, concentration of HCl, and the loading of BTBAC. Furfural and 5-(chloromethyl)furfural (CMF) have also been coproduced from a mixture of pentose and hexose sugars. Under optimized conditions (100 °C, 3 h, 20.2percent HCl, 10 wtpercent BTBAC), CMF and furfural were isolated in 17percent and 53percent yield, respectively, from a mixture of glucose and xylose. In addition, levulinic acid was isolated from the aqueous layer in 31percent yield.

Full text of DOI:10.1016/j.carres.2020.108105

Journal Pre-proof
Hydrochloric acid-catalyzed coproduction of furfural and 5-(Chloromethyl)furfural
assisted by a phase transfer catalyst
Navya Subray Bhat, Nivedha Vinod, Sharath Bandibairanahalli Onkarappa, Saikat
Dutta
PII:
S0008-6215(20)30154-3
DOI:
https://doi.org/10.1016/j.carres.2020.108105
CAR 108105
Reference:
To appear in: Carbohydrate Research
Received Date: 9 March 2020
Revised Date: 2 July 2020
Accepted Date: 14 July 2020
Please cite this article as: N.S. Bhat, N. Vinod, S.B. Onkarappa, S. Dutta, Hydrochloric acid-catalyzed
coproduction of furfural and 5-(Chloromethyl)furfural assisted by a phase transfer catalyst, Carbohydrate
Research (2020), doi: https://doi.org/10.1016/j.carres.2020.108105.
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Graphical Abstract

Hydrochloric Acid-Catalyzed Coproduction of Furfural and 5-(Chloromethyl)furfural
Assisted by a Phase Transfer Catalyst
Navya Subray Bhat, Nivedha Vinod, Sharath Bandibairanahalli Onkarappa, and Saikat
Dutta*[a]
a Department of Chemistry
National Institute of Technology Karnataka (NITK), Surathkal, Mangalore-575025,
Karnataka, India
E-mail: sdutta@nitk.edu.in
Abstract: Furfural has been produced in 53% isolated yields from D-xylose within an
aqueous HCl-1,2-dichloroethane biphasic reaction mixture using benzyltributylammonium
chloride (BTBAC) as a phase transfer catalyst. The use of BTBAC noticeably improved the
yield of furfural compared to that in the control reaction. The reaction was optimized on the
reaction temperature, duration, concentration of HCl, and the loading of BTBAC. Furfural
and 5-(chloromethyl)furfural (CMF) have also been coproduced from a mixture of pentose
and hexose sugars. Under optimized conditions (100 °C, 3 h, 20.2% HCl, 10 wt% BTBAC),
CMF and furfural were isolated in 17% and 53% yield, respectively, from a mixture of
glucose and xylose. In addition, levulinic acid was isolated from the aqueous layer in 31%
yield.
Keywords: biomass, 5-(chloromethyl)furfural, furfural, phase transfer catalyst, renewable
synthesis.
1 Introduction
The renewable and carbon-neutral character of biomass makes it an environment-friendly
alternative to the exhaustible fossilized carbon sources for the production of liquid
transportation fuels and chemicals [1]. A major challenge for the conversion of biomass into
fuels and chemicals is the depolymerization and selective deoxygenation of the complex,
oxygen-rich biopolymers [2]. The chemocatalytic value addition of biomass in the biorefinery
realm is of particular interest since they are fast, selective, biomass agnostic, and could
potentially be integrated into the existing petrorefinery infrastructure [3]. Feedstock plays a
pivotal role in the commercial feasibility of a biorefinery. In this regard, carbohydrates in the
form of cellulose and hemicellulose constitute more than half of the total mass of terrestrial
and algal biomasses [4]. Glucose receives special interest since it is abundant, inexpensive,

and the monomer of the most abundant biopolymer cellulose. Hemicellulose is the second
most abundant polysaccharide (20-35% of lignocellulosic biomass), where xylose is one of
the major constituents [5]. The acid-catalyzed dehydration of sugars is an elegant strategy
that allows selective removal of the oxygen atoms and excessive functionalities from the
sugar moiety in the form of water. Since the transformations do not involve any carbon-
carbon bond scission reactions, the carbon efficiency is quantitative. The production of
furfural and 5-(hydroxymethyl)furfural (HMF) from xylose and glucose, respectively, are
known for over a century, and the commercial production of furfural was realized in as early
as 1921 [6]. Furfural and HMF retain some of the reactive functional groups of the parent
sugars that can be exploited for downstream value-addition pathways [7]. Both furfural and
HMF have been demonstrated as renewable chemical building blocks that can be
transformed into fuels, specialty chemicals, and materials by selective chemical
transformations [8, 9]. Over the past three decades, hundreds of publications and patents
have been dedicated to the production and application of furfural and HMF, and several
comprehensive reviews have summarized these works [10-12]. Although HMF is routinely
obtained in high yields from simple sugars like fructose, the high-yielding production of HMF
from cellulosic biomass often requires special reaction conditions [13]. The commercial
production of HMF is plagued by the hydrophilicity and inherent instability of HMF in
aqueous acid [14]. The coproduction of HMF and furfural from a mixture of pentose and
hexose sugars, as well as lignocellulosic biomass, is also reported [15]. In recent years, 5-
(chloromethyl)furfural (CMF), a hydrophobic congener of HMF, has been explored as a
functional substitute for HMF. CMF can be synthesized in high isolated yields even from
carbohydrates and untreated cellulosic biomasses, alike [16]. CMF has been demonstrated
as a renewable chemical platform for the synthesis of a diverse range of specialty chemicals
[17]. Therefore, an efficient coproduction of furfural and CMF from the mixture of glucose
and xylose could work as a working model for terrestrial and algal biomasses. An aqueous-
organic biphasic reaction is frequently adopted for the production of furanics from biomass
[18]. The idea is to sequester the furanic molecules into the organic phase as soon as they
form and slow down their decomposition in aqueous acid. Benzyltributylammonium chloride
(BTBAC) is a known phase transfer catalyst (PTC) for various organic synthetic applications
[19]. In a recent report, we had shown that CMF could be prepared from various
carbohydrates in concentrated hydrochloric acid-1,2-dichloroethane (DCE) biphasic reaction
using BTBAC) as a PTC [20]. The use of BTBAC afforded higher isolated yields of CMF
when compared to the control reactions. We reasoned that the hydrophobic environment
created by the PTC shielded the reactive intermediate species during the formation of CMF,
thereby increasing the selectivities and yields of the same. We envisaged that the process
could also be extended for the production of other furanic molecules such as furfural. In this

work, we report the production of furfural from biomass-derived pentose sugars like xylose in
aq. HCl-DCE biphasic system using BTBAC as a phase transfer catalyst. The reaction was
optimized on reaction temperature, duration, loading of BTBAC, and concentration of HCl.
The optimized reaction condition was then applied for the one-pot coproduction of furfural
and CMF from a mixture of pentose and hexose sugars such as glucose and xylose
(Scheme 1). The 20.2%HCl-DCE biphasic reaction was conducted in a glass pressure
reactor in the presence of BTBAC (10 wt.%) as additive. The azeotropic concentration of HCl
was used to allow straightforward recovery of the aqueous acid by distillation while
minimizing the decomposition of BTBAC.
O
O
O
D-Glucose
+
aq. HCl-DCE
O
O
OH
+
H
Cl
H
+
PTC, ∆
D-Xylose
O
Furfural
CMF
Levulinic acid
Scheme 1. Coproduction of CMF and furfural from a mixture of glucose and xylose in the
presence of a phase transfer catalyst.
2 Results and Discussion
Initially, the preparation of furfural from xylose was attempted using aqueous HCl (20.2%)-
DCE biphasic medium. The use of azeotropic HCl helped to avoid degassing of hydrogen
chloride during the reaction. The acid catalyst could be recovered conveniently via distillation
without altering the concentration. DCE was chosen as the solvent due to its availability in
bulk, intermediate boiling point, chemical inertness under the reaction conditions used, and
recoverability by distillation. The loading of xylose was initially fixed at 5 wt.% compared to
the amount of aqueous HCl used. In a typical reaction, xylose was dissolved in aqueous HCl
(20.2%) taken in a sealed round-bottomed glass pressure reactor fitted with a Teflon screw-
top and a magnetic stir bar. The solvent (i.e., DCE) and BTBAC (10 wt.% of xylose) were
added before the reactor was sealed. The reactor was then placed in a pre-heated (100 °C)
oil-bath and stirred magnetically at 400 rpm for 3 h. The reactor was then cooled down to
RT, opened, and the mixture was transferred into a separatory funnel. The DCE layer was
separated, dried, and distilled off under reduced pressure to isolate furfural. The control
reaction was carried out under the same reaction conditions, except no BTBAC was added.
The reaction with BTBAC as the PTC afforded furfural in 53% isolated yield, whereas the
yield of furfural in the control reaction was only 34% (Table 1). The mass balance is the
soluble and insoluble humin formation, which was noticeably more in the control reaction.

Table 1. Preparation of furfural from D-xylose using various Brønsted acid catalysts.
Reaction Conditions: Xylose (1 g), aqueous acid (20 mL), DCE (40 mL), BTBAC (0.1 g)
Reaction
Yield
(%)[b]
53
S/N
1
Catalyst
conditions
HCl (20.2%,
6M)
100 °C, 3 h
100 °C, 3 h
100 °C, 20 h
100 °C, 3 h
100 °C, 5 h
(34)[c]
35
2
H2SO4 (6M)
(20)
19
CF3CO2H
(6M)
3[a]
(08)
28
CH3SO3H
(6M)
4
(21)
22
5
H3PO4 (6M)
(15)
[a] The yield of furfural was poor (<5%) after 3 h. [b] Isolated yield. [c] The yield of furfural in
the control reaction.
Various frequently used Brønsted acid catalysts were examined for the preparation of
furfural as described in Table 1. When sulfuric acid was used as the acid catalyst (entry 2),
furfural was isolated in 35% yield within 3 h at 100 °C, whereas the control reaction provided
only 20% yield under identical conditions. Methanesulfonic acid (entry 4) provided marginally
lower yield, but the positive effect of BTBAC on the furfural yield was still pronounced.
Weaker acids, like trifluoroacetic acid (entry 3) and orthophosphoric acid (entry 5), provided
lower yields of furfural even after significantly longer reaction time. Interestingly, the yields of
furfural in the control reactions were still noticeably lower. The results may be explained by
the continuous decomposition of furfural during the course of the reaction, even when the
conversion of xylose remain low.
Since aqueous hydrochloric acid (20.2%) was found to be the most effective catalyst, the
reaction was further optimized on other reaction parameters for the best selectivity and yield
of furfural starting from xylose.

60
50
40
30
20
10
0
53
50
49
48
35
80
90
100
110
120
Reaction temperature (°C)
Figure 1. The effect of reaction temperature on furfural yield. Reaction conditions: xylose (1
g), BTBAC (0.1 g), DCE (40 mL), aq. HCl (20.2%, 20 mL), 3 h.
At first, the effect of reaction temperature was studied by keeping the other reaction
parameters unchanged. When the reaction was conducted at 80 °C, only a 35% yield of
furfural was isolated (Figure 1). Increasing the reaction temperature to 100 °C increased the
yield significantly to 53%. Further increase in temperature marginally decreased the yield of
furfural. The results may be explained by the incomplete conversion of xylose at lower
temperatures, whereas faster decomposition at higher temperatures. When the reaction was
carried out under reflux, furfural was isolated in only 23% yield after 3 h, which increased up
to 29% after 6 h of reaction time. The control reaction under identical conditions (reflux, 3 h)
provided 14% yield of furfural.
55
50
45
40
35
30
25
20
4M
6M
8M
Concentration of aq. HCl
Figure 2. The effect of aqueous HCl concentration on furfural yield. Reaction conditions:
xylose (1 g), BTBAC (0.1 g), DCE (40 mL), aq. HCl (20 mL), 100 °C, 3 h.

The effect of the concentration of aqueous HCl on the isolated yield of furfural was then
explored. The use of 4M HCl provided only 42% of furfural (100 °C, 3 h) (Figure 2). The
result may be rationalized by the incomplete conversion of xylose at lower acid
concentration. Increasing the acid concentration to 8 M also lowered the yield of furfural to
45% compared to 53% in 20.2% (6 M) HCl. The result may be explained by the instability of
the BTBAC catalyst at higher concentrations of HCl [21].
55
50
45
40
35
30
25
20
2
3
4
Reaction time (h)
Figure 3. Effect of the duration of reaction on the yield of furfural. Reaction conditions:
xylose (1 g), BTBAC (0.1 g), DCE (40 mL), HCl (20.2%, 20 mL), 100 °C.
The effect of the duration of reaction, keeping the reaction temperature fixed at 100 °C
(Figure 3). After 2 h of reaction time, the yield of furfural was only 40% due to incomplete
reaction. However, prolonged reaction time (ca. 4 h) also lowered the yield of furfural due to
more decomposition. Though the use of less BTBAC (5 wt.%) led to lower yield of furfural
(ca. 48%), higher loadings of BTBAC (10 wt.%) did not show any discernible improvement in
the yield of the same. The formation of insoluble humin as black solid was found to be only
0.036 g for 1 g of xylose under the optimized conditions.
The optimized reaction was then applied for the coproduction of CMF and furfural starting
from a mixture of xylose and various hexoses. The products could be separated by column
chromatography in the laboratory scale or by fractional distillation in a larger scale. The
1
relative ratio of products was also calculated by the integration of peaks in the H-NMR
spectrum. The isolated mass of CMF and furfural conformed well to the NMR data. When a
mixture of glucose and xylose was used as the feedstock, CMF, and furfural were isolated in
17% and 53% yields, respectively, under the optimized conditions (Table 2, entry 1).
Interestingly, when glucose was used alone, CMF was isolated in similar yields (ca. 18%).

Therefore, the use of physical mixture of pentose and hexose sugars did not affect the
individual yields of furfural and
Table 2. Coproduction of furfural and CMF. Reaction conditions: HCl (20.2%, 20 mL), DCE
(40 mL), 100 °C, 3 h, 0.5 g of each substrate, 0.1 g BTBAC.
Yield (%)
Yield (%)
without
with BTBAC
S/N
Substrate
BTBAC
CMF Furfural CMF Furfural
1[a] Glucose+Xylose
17
16
16
15
16
15
53
51
52
49
48
48
12
10
12
11
11
10
34
34
35
32
33
32
2
3
4
5
6
Sucrose+Xylose
Fructose+Xylose
Inulin+Xylose
Starch+Xylose
Cellulose+Xylose
[a] LA was isolated from the aqueous layer in 31% (with BTBAC) and 15% (without BTBAC)
yields, respectively.
CMF. Due to low concentration of chloride ion, the yields of CMF remained low for all the
hexoses studied. When NaCl was used in excess, the yield of CMF improved to 29% (with
BTBAC). We envisioned that instead of forming CMF, the HMF intermediate would likely
tranform into LA in the aqueous layer. Extraction of the aqueous acid with ethyl acetate
afforded 31% of LA in the reaction using BTBAC. The yields of CMF and furfural in the
control reaction were 12% and 34%, respectively. When the combination of sucrose and
xylose was used (entry 2), CMF and furfural were isolated in 16% and 51% yields. The
combination of fructose and xylose (entry 3) provided comparable yields. The yields of CMF
were nearly same for all the polymeric carbohydrates (entry 4-6) and higher than those in the
control reactions. Interestingly, furfural was isolated in similar yields from all the mixtures of
sugars examined. The insoluble humin formation is noticably higher in the control reaction
compared to reactions using BTBAC. For example, the mixture of glucose and xylose (entry
1) produced 0.025 g of insoluble humin as black solid compared to 0.052 g of the same in
the control reaction. The BTBAC catalyst was separated from the product by column
chromatography (Silica Gel 60-120 mesh) eluting with 20% MeOH in CHCl3. Alternatively,
selective dissolution of the furanics in hot petroleum ether (60-80 °C) left behind BTBAC in
excellent mass balance (>95%). The use of recovered BTBAC for the preparation of furfural
or for the coproduction of CMF and furfural did not show any noticeable change in isolated

yields up to three consecutive cycles. The chemical stability of BTBAC was confirmed by
NMR spectrocpy and no perceptible chemical decomposition was observed.
3 Experimental Section
3.1 Production of furfural from xylose
Xylose (1 g) was taken in a 100 mL round-bottomed glass pressure reactor fitted with a
Teflon screw-top. To this, aqueous HCl (20.2%, 20 mL), DCE (40 mL), and BTBAC (0.100 g)
were added. A magnetic stirring bead was added, and the pressure reactor was sealed. The
reactor was placed in a pre-heated oil bath (100 °C) the reaction mixture was magnetically
stirred continuously for 3 h. After the reaction, the reactor was removed from the oil bath and
cooled down to room temperature. The reactor was opened, and the reaction mixture was
transferred into a separating funnel. The DCE layer was separated, and the aqueous phase
was extracted with fresh DCE (2×10 mL). The DCE layers were combined, dried over
anhydrous Na2SO4, and passed through a plug of silica gel (60-120 mesh). Evaporation of
DCE in a rotary evaporator under reduced pressure provided furfural as light yellow oil
(0.340 g, 53%). The control reactions were performed as described above, except no
1
BTBAC was used. The FTIR and H-NMR spectroscopy confirmed the identity and purity of
furfural. The FTIR and 1H-NMR spectroscopy confirmed the identity and purity of furfural. 1H-
NMR (CDCl3, 300 MHz) δ (ppm): 9.67 (s, 1H), 7.70 (s, 1H), 7.27 (d, 1H, J = 3.6 Hz), 6.61 (d,
13
1H, J = 3.6 Hz); C{1H}-NMR (CDCl3, 75 MHz) δ (ppm): 177.8, 152.9, 148.1, 121.2, 112.6;
FTIR (ATR, cm-1): 3133, 2924, 2852, 2811, 1669, 1463, 745.
3.2 Coproduction of CMF and furfural from the mixture of glucose and xylose
Xylose (0.500 g) and glucose (0.500 g) were taken in a 100 mL round-bottomed glass
pressure reactor fitted with a Teflon screw-top. To this, aqueous HCl (20.2%, 20 mL), DCE
(40 mL), and BTBAC (0.100 g) were added. A magnetic stirring bead was added, and the
pressure reactor was sealed. The reactor was placed in a pre-heated oil bath (100 °C) the
reaction mixture was magnetically stirred continuously for 3 h. After the reaction, the reactor
was removed from the oil bath and cooled down to room temperature. The reactor was
opened, and the reaction mixture was filtered through a filter paper. The filtrate was
transferred into a separating funnel. The DCE layer (bottom) was separated, and the
aqueous phase was extracted with fresh DCE (2×10 mL). The DCE layers were combined,
dried over anhydrous Na2SO4, and evaporated in a rotary evaporator under reduced
pressure to provide the crude mixture of CMF and furfural as a brown oil. The components
were separated by column chromatography (Silica gel) using a mixture of petroleum ether

and ethyl acetate as eluent. Evaporation of the solvent produced furfural (0.170 g, 53%) and
CMF (0.068 g, 17%) as a light yellow oil. The control reactions were performed as described
1
above, except no BTBAC was used. The H-NMR spectrum of the mixture of CMF and
furfural showed the same molar ratio of products as calculated by the integrals of the
relevant peaks of each compound.
3.3 Isolation of Levulinic Acid:
The aqueous layer was filtered through a filter paper, and the filtrate was saturated by
adding solid sodium chloride. The saturated solution was cooled and extracted with ethyl
acetate (6×20 mL). The ethyl acetate layers were combined, dried over anhydrous Na2SO4,
and evaporated in a rotary evaporator under reduced pressure to yield a brown liquid. The
liquid was chromatographed over silica gel (60-120 mesh) using diethyl ether as eluent.
Evaporation of the solvent provided levulinic acid (LA) as light yellow oil (0.099 g, 31%). 1H-
NMR (CDCl3, 300 MHz) δ (ppm): 2.75 (t, 2H, J = 6.3 Hz), 2.62 (t, 2H, J = 6.3 Hz), 2.20 (s,
3H); 13C{1H}-NMR (CDCl3, 75 MHz) δ (ppm): 206.6, 177.8, 37.7, 29.8, 27.7; FTIR (ATR, cm-
1): 3163, 2923, 1702, 1236, 1161.
3.4 Quantification of Humin:
The aqueous layer was passed through a filter paper, and the filter paper was washed with
excess distilled water. The filter paper was dried in a hot-air oven at 60 °C till a constant
weight was achieved.
3.5 Recovery and Characterization of BTBAC:
After isolating CMF from the silica-gel column, the column was eluted with 20%
methanol/DCM to recover the BTBAC. A 97% recovery of BTBAC was obtained using this
technique. Alternatively, the mixture of furfural, CMF, and BTBAC was triturated with
petroleum ether under reflux. Both CMF and furfural dissolved in hot petroleum ether and
decanted, whereas BTBAC is recovered as a beige solid in near quantitative yield. The
1
recovered BTBAC was characterized by FTIR and H-NMR spectra and compared with
literature data. 1H-NMR (CDCl3, 300 MHz) δ (ppm): 7.33 (m, 5H), 4.68 (s, 1H), 3.15 (m, 6H),
2.98 (s, 1H), 1.65 (m, 6H), 1.28 (m, 6H), 0.84 (t, 9H); FTIR (ATR, cm-1): 3300, 2928, 1704,
1211, 1161.
Conclusions
The use of BTBAC afforded significantly higher yields of furfural starting from xylose when
compared to the control reaction. The insoluble humic matter formed in noticeably lower

quantities in reactions using BTBAC. CMF and furfural were coproduced from a physical
mixture of xylose and hexose carbohydrates. The yields of CMF and furfural were higher
when using BTBAC as PTC. The azeotropic concentration of HCl allowed conducting the
reaction under the refluxing condition without evaporative loss of the HCl gas. No chemical
decomposition of BTBAC was observed, and it was recovered in near-quantitative yield after
the reaction BTBAC was conveniently separated from the product mixture by
chromatography or trituration with petroleum ether.
Supporting Information Summary
The spectral data are provided as supporting information.
Acknowledgments
The authors thank TIFR hyderabad, for collecting NMR (1H & 13C) data collection.
Funding Source
This study is finantially supported by the Science and Engineering Research Board (SERB),
India for funding support under the grant number YSS/2015/001649.
Conflict of Interest
The authors declare no conflict of interest
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Graphical Abstract
Highlights
•
Furfural has been produced in good isolated yield from xylose in an aqueous-organic
biphasic batch reactor using benzyltributylammonium chloride (BTBAC) as a surface-
active agent.
•
Hydrochloric acid was found to be the most active catalyst that provided furfural in 53%
yield within 3 h at 100 °C under 10 wt% loading of xylose and 1,2-Dichloroethane (DCE)
as the organic extractant.
•
•
BTBAC improved the yield of furfural noticeably compared to the control reaction.
Furfural and 5-(chloromethyl)furfural (CMF) were coproduced starting from a mixture of
xylose and glucose. In addition, levulinic acid (LA) was isolated from the aqueous phase.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: